1. Field of the Invention
This invention relates to an industrial robot, or in particular to an industrial robot required to operate at high speed for a long period of time.
2. Description of the Related Art
FIG. 7 is a perspective view showing three power transmission units of a conventional industrial robot. As shown in FIG. 7, power transmission gears 124, 125, 126 are arranged in that order coaxially on three power transmission units 121, 122, 123, respectively, arranged concentrically with each other. The three power transmission gears 124, 125, 126 are engaged by motor gears 131, 132, 133 of motors 151, 152, 153, respectively. The motors 151, 152, 153 have the same rating. As can be seen from FIG. 7, the motor gears 131, 132, 133 are arranged substantially equidistantly along the periphery of the power transmission gears 124, 125, 126.
Japanese Unexamined Patent Publication No. 2001-273037 discloses a servo control system for driving a single movable member with two motors. The position control device of the servo control system includes, for each motor, a position controller for receiving the same position command from a host control unit and processing the position deviation amount equal to the position command less the position feedback amount from the position detector thereby to output a speed command, a speed controller for receiving the speed command and processing the speed deviation amount equal to the speed command less the speed feedback amount from the speed detector thereby to output a current command, and a current controller for receiving the current command and processing the current deviation amount equal to the current command from which the current feedback amount from a sensor for detecting the motor current is subtracted, to thereby output a voltage command and activate the current amplifier in accordance with the voltage command.
In recent years, a robot has been required to be operated at high speed, and especially, in order to improve production efficiency, an increased transport ability of the robot is also required. With regard to transport ability, in addition to the high speed and high acceleration of the robot arm operation, the robot is expected to perform severe operations continuously, namely it is expected to realize operations at a high duty factor.
In the case where it is desirable to improve an operation performance of the servo control unit, the rating of the motors is usually increased. An increased motor rating, however, increases both the motor size and the rotor inertia of the motor. Further, increased motor size increases the size of transmission elements related to the motor output shaft, such as gears, resulting in increased inertia.
It is necessary for the motor to bear torque increased by the increased rotor inertia of the motor and by the increased inertia of the transmission elements is required. Even in the case where the motor rating is increased, the almost all the output of the increased motor rating is used for the torque increased by the increase of rotor inertia of the motor and the inertia of the transmission elements. This poses a problem in which it is difficult to increase the acceleration of the driving the motor output shaft. Also, by decreasing the weight of the arm, it may be possible to reduce the load. However, there is a limit to such a solution.
This invention has been achieved in view of the above, and the object thereof is to provide an industrial robot capable of high-duty operation with a comparatively compact configuration.